GTA
Airguns by Make and Model => Benjamin Airguns => Topic started by: Psipumper on May 25, 2019, 01:44:01 PM
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I wonder what the maximum power a 392 could possibly make at 14 pumps.
I have reduced the volume of my valve to 4.9cc and 14 pumps is 2100 psi in the valve. I get 954 fps with the crosman pellets and 766 with the JSB monsters.
So about 33 fpe.
My thinking as of late is the 392 should not be pumped over 14 . Just the principle of it. It’s a 392.
That being said, I did get 839fps with the monsters before reducing valve cc size.
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Wow, 33 fpe from a 392 is awesome... i’m Amazed at what some of you guys accomplish with my favorite air rifle! :o
Is your accuracy the same as stock at that level, or do the groups start opening up compared to stock?
I’m asking because i’m considering having my 392 steroided, or attempt some power modding myself.
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Wow, 33 fpe from a 392 is awesome... i’m Amazed at what some of you guys accomplish with my favorite air rifle! :o
Is your accuracy the same as stock at that level, or do the groups start opening up compared to stock?
I’m asking because i’m considering having my 392 steroided, or attempt some power modding myself.
The accuracy is the same. I shoot the 18g Jsb pellets in this one at about 800 fps. Just put it back together but It would take about 12 pumps to get that velocity.
I bought a second 392 and it seemed like a red Ryder out of the box compared to this one.
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The second 392 I bought is my favorite. It has a smaller 3.4cc valve. 8 pumps has it wound up pretty tight. I get 830 fps with the crosman 14.3 pellets. I use the 15 grain jsb in it. Haven’t even checked their velocity. Very accurate. I have ordered a tin of the new Hades pellets for it. If they are the same accuracy it should be a good combination. This gun has two .5 cc spacers that could be removed to increase valve volume to 4.4cc. This would increase the number of pumps and top power. I haven’t tried it or want to.
This gun is noticeably quieter and more efficient.
High power gun is named “K1”
Lower power gun is named “K2”
K1= 766@ 8 pumps
K2= 830@ 8 pumps
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Awesome performance!
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What's the pumping effort? Seems like you'd need hemorrhoid producing effort to get that kind of fpe from a pumper :o
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What's the pumping effort? Seems like you'd need hemorrhoid producing effort to get that kind of fpe from a pumper :o
The last few pumps are heavyweight. Kinda like the old Star Trek phaser; you don’t set it on kill that often. Usually a 10 to 12 pump permanent stun.
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Testing the gun at higher pressures with 5.75cc valve volume . I was able to get right at 40 fpe with the heaviest pellets I have. Probably would be 42 fpe with 32 grain pellets. The valve at 2400 psi would start to retain air here losing power. A balanced type valve would allow more pressure and be faster off the seat. If someone wanted to, I think 50 fpe is in reach. Is it practical or would the gun last?
No. It would just be bragging rights.
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another way to skin the cat ( from a mechanical standpoint )
sleeve the inner tube down to lets say .625 ( like a 1377 piston _
you would be able to pump over 2800 psi no issue , the effort would be less than it is now on pump 14.. The downside is youd have to complete about double the number of strokes.. Elongating the pump handle would be number one priority were i going for max power on a 39x.
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aside from that crazy idea. your numbers are the best i can remember seeing on a 39x platform/
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aside from that crazy idea. your numbers are the best i can remember seeing on a 39x platform/
Thanks Rob. That is one reason I backed the gun down to 14 pumps. To make apples to apples comparison.
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Fantastic work!
I tend to favor the bigger valves. To me the power limit of a pumper is the amount of power it can make at the safest pressure that it can take over and over again without significant wear. "Significant wear" is subjective but to me if I can go through a few tins of pellets without my piston and valve moving away from each other detectably, I'm pumping to a safe pressure. So I prefer to increase the valve volume quite a bit and then fill it to whatever I consider max pressure. Number of pumps doesn't really matter because they become easier to do, as long as I don't run out of fingers and toes to count on.
Of course, it's hard to talk about valve volume without considering barrel volume. I think that if someone were to de-solder the stock barrel and replace it with a 26-inch maximus barrel, or something .25 cal, they could really see some high muzzle energy.
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I plan on trying to convert to a blow open balanced valve for this gun. Not solely to increase power but to improve accuracy as well. A lighter hammer and spring should have less jump on release and a smoother trigger.
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I plan on trying to convert to a blow open balanced valve for this gun. Not solely to increase power but to improve accuracy as well. A lighter hammer and spring should have less jump on release and a smoother trigger.
thats brilliant and makes perfect sense.. part of the reason the 880 daisy does so much with soo little is the blow open valve design..
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Wouldn't you have a too-heavy trigger with a blow-open valve and that kind of pressure?
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This type of balanced valve is not like the Daisy 880. The rear of the poppet is vented to atmospheric pressure. If I can design it keeping the valve volume the same size is my goal. Not sure what the bias size of the balance chamber to throat diameter should be. Maybe 75% to 80% if possible.
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This type of balanced valve is not like the Daisy 880. The rear of the poppet is vented to atmospheric pressure. If I can design it keeping the valve volume the same size is my goal. Not sure what the bias size of the balance chamber to throat diameter should be. Maybe 75% to 80% if possible.
ok .. a balanced valve..when u said blow open i was thinking SSP or 880 style ( literally blow open )A balanced design would be awesome .. There was a thread where i think wyoman or gipetto ( no it was tackdriver ) made a balanced valve for his 1377 .. It was the simplified version where the stem literally exhausts back to the hammer area. I would think for this application a small pen spring creating the differential would work.. In other words the atmospheric front chamber could house a small spring, Since efficiency will not be an issue when going for full exhaust of the valve.
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to clarify.. there is no remaining force to close the valve after the shot.. SO a return spring would be needed for the next shot.. You could have an effective balance of 100% - the spring force , which generally might be 5 lbs tops.. this way youre reset for the next shot.
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to clarify.. there is no remaining force to close the valve after the shot.. SO a return spring would be needed for the next shot.. You could have an effective balance of 100% - the spring force , which generally might be 5 lbs tops.. this way youre reset for the next shot.
If you were to remove the spring, yes it would remain open. Since the rear of the poppet vents to atmosphere instead of the pressurized throat during firing, the poppet is forced to the rear or blown open when it leaves the seat.
A sufficient valve spring still has to be used to seal the poppet on the first pump as usual. My concern would be if 75% bias would apply enough additional seal pressure on the poppet seat at 2100 psi.
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to clarify.. there is no remaining force to close the valve after the shot.. SO a return spring would be needed for the next shot.. You could have an effective balance of 100% - the spring force , which generally might be 5 lbs tops.. this way youre reset for the next shot.
If you were to remove the spring, yes it would remain open. Since the rear of the poppet vents to atmosphere instead of the pressurized throat during firing, the poppet is forced to the rear or blown open when it leaves the seat.
A sufficient valve spring still has to be used to seal the poppet on the first pump as usual. My concern would be if 75% bias would apply enough additional seal pressure on the poppet seat at 2100 psi.
i see what you mean.. I think bob said 67% was always a safe bet.. and yes unlike a pcp this has to build from 1 pump, nothing will work if it wont seat during the first pump.. I CHEATED a few years ago on my second 50 caliber co2 rifle. I oringed the seat and made a huge poppet, this way almost no sealing force was required.. Granted it was an odball build but keep in mind if the valve is balanced the poppet diameter in not important ( so long as the balancing is correct
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Have you thought about making a flat-top piston? In my 1322, I made an adjustable rubber cup piston at one point, but I found that a flat-top piston with an o-ring was much easier on the linkage because it didn't need as much interference to make full pressure at higher pump counts.
Of course the 392 is different in that there seems to be less material in the nose of the valve ahead of the check. When I had mine open, I intended to stick a pin in the check valve like I do on my 13XX guns to eat dead space, but it didn't seem necessary because the check valve was so close to the air inlet hole. I don't recall if there was enough material there to completely flat top it without additional modifications.
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Have you thought about making a flat-top piston? In my 1322, I made an adjustable rubber cup piston at one point, but I found that a flat-top piston with an o-ring was much easier on the linkage because it didn't need as much interference to make full pressure at higher pump counts.
Of course the 392 is different in that there seems to be less material in the nose of the valve ahead of the check. When I had mine open, I intended to stick a pin in the check valve like I do on my 13XX guns to eat dead space, but it didn't seem necessary because the check valve was so close to the air inlet hole. I don't recall if there was enough material there to completely flat top it without additional modifications.
A flat top piston is possible but I can’t imagine much improvement over current. You are not going to trim 2 pumps and make the same power. If the oring used in the conversion were not square you could end up losing power instead. The square ones usually aren’t available in 90 duro hardness. Wear and extrusion could become an issue over time.
I did sleeve the air inlet hole to about 1 mm. I turned a wire welder tip down. Saved me from drilling the hole.
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Fantastic work!
I tend to favor the bigger valves. To me the power limit of a pumper is the amount of power it can make at the safest pressure that it can take over and over again without significant wear. "Significant wear" is subjective but to me if I can go through a few tins of pellets without my piston and valve moving away from each other detectably, I'm pumping to a safe pressure. So I prefer to increase the valve volume quite a bit and then fill it to whatever I consider max pressure. Number of pumps doesn't really matter because they become easier to do, as long as I don't run out of fingers and toes to count on.
Of course, it's hard to talk about valve volume without considering barrel volume. I think that if someone were to de-solder the stock barrel and replace it with a 26-inch maximus barrel, or something .25 cal, they could really see some high muzzle energy.
.25 caliber would be interesting. Crosman should do that. I have never seen a 397 modified.
The safe pressure for a 392 depends. My gun is 1900 to 2300. Other people have said 1800 for a stock 392. I sized my valve volume to develop this maximum pressure at the desired 14 pumps.
Since I have the pressure gauge added to the gun, I was able to do this.
If I could improve headspace more the valve cc would be larger @ 14.
One thing for sure headspace is extremely important. My 392 only made 1600 psi at 25 pumps before work. Probably close to its maximum.
About par with the rest. Seems the 392 is a good time proven design of proper proportions in need of a lot of fine tuning and blueprinting without throwing it out of balance.
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What is the barrel length on a 392?.... Maximum practical valve volume is 50% of the barrel volume when going for power.... but it takes a LOT of pumping and the efficiency is poor.... The smaller the valve the higher the efficiency, but the lower the potential FPE.... when the number of pumps is not an issue and you are pumping to a constant fill pressure....
Bob
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What is the barrel length on a 392?.... Maximum practical valve volume is 50% of the barrel volume when going for power.... but it takes a LOT of pumping and the efficiency is poor.... The smaller the valve the higher the efficiency, but the lower the potential FPE.... when the number of pumps is not an issue and you are pumping to a constant fill pressure....
Bob
Barrel is 11.8cc
Factory valve cc is 4.2 cc =36%
4.2 cc would overpressure my gun before 14 pumps. Right now I get 2100 psi @14 with a 4.9 cc valve.=41%
At 50% it would take 20 pumps to get 2100 psi.
Since I had a desired number of pumps of 14 at maximum operating pressure. I sized the valve cc @ 4.9cc to get this. This should be the optimal valve size for maximum power at 14 pumps.
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What is the barrel length on a 392?.... Maximum practical valve volume is 50% of the barrel volume when going for power.... but it takes a LOT of pumping and the efficiency is poor.... The smaller the valve the higher the efficiency, but the lower the potential FPE.... when the number of pumps is not an issue and you are pumping to a constant fill pressure....
Bob
Barrel is 11.8cc
Factory valve cc is 4.2 cc =36%
4.2 cc would overpressure my gun before 14 pumps. Right now I get 2100 psi @14 with a 4.9 cc valve.=41%
At 50% it would take 20 pumps to get 2100 psi.
Charles you must have strong shoulders pumping that thing to 2100 (-'
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Thanks for that info.... so about 19".... Around 40% sounds like a good place to be.... but don't forget you could get the same FPE with a larger valve and a bit less pressure, so maybe 18 pumps and 1900 psi but with a 5.9 cc valve.... and still have the option of going to 20 pumps without overstressing the gun and get even more FPE.... All I'm saying is that 14 pumps is just a number, it has to be combined with the valve volume to tell you the stress you are putting on the parts....
If I had a 5.9 cc valve I would probably end up tuning it to retain air and get two equal shots.... That's the cool thing about pumpers, so many ways to tune them....
I agree, 2100 psi would be quite hard to pump, and very hard on a stock 392 linkage.... I used 1800 psi for a maximum and usually only 1500 with my .25 cal Disco based Carbine with the stock 392 linkage.... and 2000 psi for the maximum and 1800 for most shooting with a Steroid linkage.... I was worried about the stock linkage failing so never even tried 2000 psi.... That was hard enough for me to pump, and the Disco tube is only 0.74" ID, slightly smaller than a 392 tube....
Bob
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I've found that with pumpers, I can predict the effect of small changes in pressure and volume with a couple of formulas. Increasing pressure at the same volume gives an increase in velocity of roughly sqrt(pressureA/pressureB) and increasing volume at the same pressure gives an increase of roughly sqrt(sqrt(VolumeA/VolumeB) [4th root of the VolA/VolB)]. Likewise, increasing volume with a proportionate decrease in pressure results in a loss of velocity equal to sqrt(sqrt(VolA/VolB). The reverse is true for decreasing volume. The catch is that this only works when you can actually measure (or at least accurately estimate) the pressure. In a multi-pump, you can't just assume that when you double valve volume you need twice as many pumps to reach the same pressure - it will actually be less than that because % of head space is lower if the valve volume is increased. The reverse is true if you make the valve smaller - the cumulative effect of head space losses will become unmanageable at a lesser number of pumps. I believe that this is why stock guns can actually gain FPS in some cases with a larger valve and no other changes - they have so much head space and they have so much built-in flex (head space that increases with pressure) that they will perform better pump for pump with a larger valve after a certain number of pumps.
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Yes, with constant volume (and everything else), the velocity should increase with the square root of the pressure, because the FPE is (at least in theory) proportional to the pressure.... and of course FPE uses the square of the velocity in its calculation....
Bob
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With the math then we could calculate to some degree what 50 fpe would require?
I got 839 fps with 25.3 grain with 5.75 cc valve @ 2300 psi. What would be your prediction of pressure needed for a 6.5 cc valve to make 50fpe.
Assuming the hammer strike could open the valve. Maybe a retractable bolt probe and other refinements added could assist.
32 grain pellet?
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So that's 39.7 FPE with 5.75cc and 2300 PSI. That same pressure with 6.5cc should get you 42.2 FPE (39.7*sqrt(6.5/5.75)). To get to 50... 50/42.2 * 2300 = 2725 PSI
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Well, I made a couple of .25 cal pumpers, and my Millennium Pumper would do one shot at 50 FPE with a JSB 25.4 gr. pellet, 2 equal shots at 45 FPE, or 3 shots within a narrow ES at 40 FPE.... These were achieved at 1800 psi with a valve volume of 28.5 cc.... The barrel length was 23.8".... The gun required 80 pumps to fill to 1800 psi from empty, which is why I added a Foster fitting for initial fill.... When tuned for 3 shots of 40 FPE, it took 11 pumps to refill after 1 shot, 24 after 2, and 40 to refill to 1800 after 3 shots.... After my experience with that gun, I would never even consider trying to build a .22 cal pumper that could achieve 50 FPE....
Having said that, if I put your results into Lloyd's Internal Ballistics spreadsheet, and assume a dump shot (no retained air)…. and then change one thing at a time, going to 6.5 cc increases the velocity with the 25.3 gr. pellet to 855 fps at 2300 psi (41.1 FPE).... Increasing the pressure to 3000 psi should allow a 25.3 gr. pellet to hit about 949 fps (50.6 FPE)…. At 2300 psi with a 32 gr. pellet (6.5 cc valve) you should get 779 fps (43.2 FPE)…. Increase that to 2800 psi with the 32 gr. you should hit about 845 fps (50.8 FPE)…. It is possible that with full bore-area porting and a retractable bolt you may be able to achieve 50 FPE at an even lower pressure.... The above calculations were done with a 19" barrel length....
To achieve those pressures would, IMO, require a much smaller diameter pump tube, and a lot of pumps, however.... Dropping down from a 7/8" OD Disco tube to a 3/4" OD 1322 diameter tube would require the same pumping effort at 3000 psi as the Disco tube at 2100.... I wouldn't want any more effort than that, as filling my Millennium Pumper to 1800 was hard enough, and 2000 was more effort than I wanted to expend, except for a couple of tests.... With the same stroke length, you would need more than twice as many strokes to reach 3000 psi with the small tube as 2100 with the larger one.... Bear in mind that the 392 tube is even larger ID than the Disco tube....
Could it be done, I have no doubt.... providing you properly engineered all the components to stand the strain....
Bob
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Bob, do you have a link to Lloyd's spreadsheet? When I was contemplating my last build, I searched high and low for it, but I neglected to ask...
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The way I calculate the effect of a change in pellet weight on FPE is to compare the mass of each pellet to the combined mass of the respective pellet and the air charge. Mass of a compressed gas = (pressure * volume) / temperature.
5.75cc air at 14.7 PSI and 300 Kelvin = 0.1 grain
5.75cc air at 2300 PSI and let's say 350 Kelvin = 13.4 grain
So with a 25.3 grain pellet, the mass of the pellet is 65% of the total mass of compressed air + lead, which is 38.7
With a 32 grain pellet, the mass of the pellet rises to 32 and the total mass rises to 45.4. Now the pellet is 70.5% of the total load compared to 65% before. This is an 8% [70.5/65] increase in the proportion of the energy that goes towards accelerating the lead rather than the air.
So with the heavier pellet, you have the potential for 8% more FPE at your current pressure and volume.
Taking this calculation to an extreme... you SHOULD be able to hit 50 FPE as your guns stands with a 64.1 grain slug at about 592fps, but this is all theoretical of course. The bigger the change, the greater the effect of factors that my simple calculations don't account for ;)
64.1/(64.1+13.4) = 82.7%
82.7/65 is a 27.2% increase. 39.3 FPE actual * 1.272 = 50.0 FPE predicted
The easiest way to test this would be to glue two 32 grainers together and see what actually happens.
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Thanks Tom and Bob for crunching the numbers and thoughts on that.
I am getting closer to my optimal version of the 392. I will be sure to create a more detailed thread of it here when it’s done.
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Tom, Lloyd's spreadsheet is proprietary, and I am one of the fortunate few to have a copy.... partly because I assisted him with suggestions to improve it....
Not all of the air in the valve is accelerated out the barrel with the pellet.... If the valve is 40% of the bore area, over 31% of it is still in the valve at the instant of pellet exit (the residual pressure at pellet exit is 725 psi), and some of that has barely left the valve, so it's velocity is not the same as the pellet.... If I take the original setup with a 5.75 cc valve at 2300 psi, and substitute a 64.1 gr. slug for the 25.3 gr. pellet I only get 564 fps (45.3 FPE), not 50.... According to Lloyd's spreadsheet, even a 200 gr. bullet would only get to 48.5 FPE at 2300 psi with the 5.75 cc valve....
In practical tests, the FPE does not increase forever with bullet weight, it peaks at some weight, different of course for every gun....
Bob
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Ah, understood. It sounds like his spreadsheet has evolved into a very effective model. It's great to have guys like you as a resource.
As I look back at my own crude model, I will take note of the fact that using the full air mass contained in the valve of 13.4 grains is much too high for the reasons you outlined. Interestingly, if I account for your figure of air remaining in the valve and reduce my estimated air charge mass by 31% to 9.2 grains, the numbers DO get a lot closer to the numbers you got from the spreadsheet. [64.1/(64.1+9.2)]/[25.3/(25.3+9.2)] * 39.3 = 46.8 FPE. Plugging in 9.2 grains of air with a 200 grain slug would give a figure of 51.2 FPE. So it's still too high, but almost close enough for a satisfactory back-of-the-napkin type calculation. Like you said, some of the air that has left the valve has "barely left the valve." Air is a fluid, and fluids do funny things.
I think that if the model developed in the Internal Ballistics spreadsheet were ever made commercially available in a user-friendly format, there would be a lot of us lining to buy it!
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Try using half the mass of air that has exited the valve and see what you get.... IIRC, Lloyd's spreadsheet adds 1/2 the air mass "previously" released by the valve to the mass of the pellet at each interval.... but I'm too lazy to go into the code to confirm that.... ::)
I'm pretty sure the reason for using half the mass is that the air is expanding behind the pellet, so the air immediately following the pellet is travelling at pellet velocity, but the air just coming out of the valve is not requiring any real work done on it (at that moment).... ;)
Bob
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1/2 the air mass gets me 45.0 for 64.1 grain and 48.1 for 200 grain - you're right, that's very close! I also guessed at an internal temperature of 350K (170F) in my calcs so there is some wiggle room there as well.
Speaking of temperature... Let's imagine what would happen if someone equipped the valve with custom high temperature Viton seals, a non-flammable silicone-based lubricant, a thermistor and digital pressure sensor both sending data to a micro controller programmed to keep the valve below, say, 3000 PSI and 600K, while regulating the output of a heating element on the valve powered by a couple of high-drain safe chemisty Li-MN 18650 batteries... FPE could be nearly doubled with digitally-controlled electronic heating of the valve. Just put 10 pumps in and wait for the oven timer to go off ;)
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The air charge in a pumper cools on sitting, of course.... My Uber-Carbine, which was set up for two shots at 600 fps using 18 pumps needed topping up to 20 (ie 2 more pumps) if you let it sit to cool, as it would in hunting....
I dunno about the idea of heating the air to raise the pressure after pumping.... It would certainly slow down the rate of fire, no?.... I'm thinking no follow-up shots....
Bob
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No follow up shot - unless you tuned the gun to conserve air and used the tech to get two full power shots instead of one double-power shot. I wonder how long it would take to heat the air in the valve with 50-100 watts? The first shot would take the longest to prepare because first the housing itself would have to get up to operating temp. There would also have to be some fins inside the valve to transfer heat to the air more rapidly.
Sorry, Charles, I didn't mean to derail your thread - I am eagerly awaiting the next phase for your 392. I think this is one platform that hasn't gotten as much attention as it deserves.
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Works that way some times. I think we have answered the question of max power. 38 to 42 fpe. Another question of interest would be the .177 caliber version and the hypothetical .25 caliber.
Would calculations using the baseline efficiency of my gun at 40 fpe (2300psi and 5.75 cc) be usable for that?
My guess would be in .177 -1025@8 pumps with 3.4 cc valve.
In .25 caliber, I don’t know. They make a pellet the same weight (25.4)
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Great thread guys . I absolutely love my steroid pumper , too bad it only gets used as a truck gun for fall bird hunting . Pretty awesome that you got that kind of power from this platform . If you can get your hands on some 30gn BBT cast bullets they are worth a try . That’s all I feed mine with and I haven’t missed a bird with it yet . ;)
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Charles, it is of course easier to get FPE in .25 cal and harder in .177.... because of the difference in the base area of the pellet that the air pushes on.... In addition, you should use a smaller valve volume in .177 and larger in .25.... For a 50% valve volume, with a 19" barrel that works out to 3.8 cc in .177 cal, 5.9 cc in .22 cal and 7.6 cc in .25 cal.... A stock Benji valve, at 4.2 cc, is already large enough for a .177....
My .25 cal Disco Carbine, with a 20" MRod barrel (16 cc) and a stock Benji 392 pump linkage in a Disco tube, got 750 fps (32 FPE) with the 25.4 gr. Kings at 1500 psi with a 6.1 cc valve (pump and dump, 38% valve volume)…. That was the extent of my testing with a small valve, after that I changed to a 27cc valve using retained air for multiple shots.... At maximum hammer spring preload I hit 40 FPE at 1500 psi, but the valve was retaining enough air (1000 psi) for a 760 fps 2nd shot with 550 psi left after the 2 shots.... However, the velocity of the 1st shot appeared to be plateauing, so I don't think more hammer strike would have increased the velocity of the first shot any more, just wasted more air....
Bob
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Charles, it is of course easier to get FPE in .25 cal and harder in .177.... because of the difference in the base area of the pellet that the air pushes on.... In addition, you should use a smaller valve volume in .177 and larger in .25.... For a 50% valve volume, with a 19" barrel that works out to 3.8 cc in .177 cal, 5.9 cc in .22 cal and 7.6 cc in .25 cal.... A stock Benji valve, at 4.2 cc, is already large enough for a .177....
My .25 cal Disco Carbine, with a 20" MRod barrel (16 cc) and a stock Benji 392 pump linkage in a Disco tube, got 750 fps (32 FPE) with the 25.4 gr. Kings at 1500 psi with a 6.1 cc valve (pump and dump, 38% valve volume)…. That was the extent of my testing with a small valve, after that I changed to a 27cc valve using retained air for multiple shots.... At maximum hammer spring preload I hit 40 FPE at 1500 psi, but the valve was retaining enough air (1000 psi) for a 760 fps 2nd shot with 550 psi left after the 2 shots.... However, the velocity of the 1st shot appeared to be plateauing, so I don't think more hammer strike would have increased the velocity of the first shot any more, just wasted more air....
Bob
Ok. A hot .177 or .20 caliber is no problem. The power ratio to a 392 would be the same as it has been in the platform. The .25 looks like it could have issues in a pumper. Just not enough pressure and volume to do it right.
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A bit more barrel length, and a bit more pressure, will give satisfactory results in .25 cal.... My Millennium Pumper did a single shot of 50 FPE, 2 equal shots of 45 FPE, or three of 40 FPE, using retained air, a 25" barrel, 28.5 cc valve, and 1800 psi.... I don't know how large a valve you can get inside a 392, that will be one of the limiting factors, along with the pressure you can pump to without buckling the pump linkage....
Just increasing the caliber from .22 to a .25 cal 19" barrel should take that 39 FPE 392 with the 5.75 cc valve to 45 FPE (nearly 900 fps with the same weight 25.4 gr pellet) at 2300 psi.... If you can squeeze in a 7.6 cc (50%) valve, you should be tickling 50 FPE at the same pressure.... A lot more pumping to get there, of course....
Bob
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Charles, you've done the 3 o-ring mod, right? I wonder if you could drill out or cut away some of the center of the valve and use the tube walls to hold the air? That would sure increase the volume.
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The guys on the Yellow Forum used to "Skeletonize" the 392 valves to do just that.... I don't know what volume they achieved, though.... Extreme version....
(https://www.mcarterbrown.com/forums/attachments/sheridan/29715d1327889976-airgunner-solution-valve-volume-cartridge-valved-sheridan-skel1.jpg)
You must retain the check valve portion separately, obviously, in this version.... If you used a "captive O-ring" check valve, where the only moving part is the O-ring, held on a flat head screw with a groove in the shank, that is threaded into the air entry port, you could shorten the front housing to just behind the new O-ring groove....
Bob
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I didn’t realize the velocity in .25 would improve that much. Looking at the available pellets weights to slow it back down to about 850 would add more fpe.
I would be happy at 5.75 cc .
I think the crosman custom shop should offer the barrel assembly. Model 395PA sounds good.
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Here is the captive O-ring check valve.... The idea came from Steve in NC on the Green Forum....
(https://i378.photobucket.com/albums/oo221/rsterne/Parts%20for%20Sale/O-ring%20Check%20Valve_zpsvdevebgt.jpg) (http://s378.photobucket.com/user/rsterne/media/Parts%20for%20Sale/O-ring%20Check%20Valve_zpsvdevebgt.jpg.html)
The flat head screw is threaded into the (tapped) air intake hole in the nose of the valve.... It has a small groove cut along one side of the threads for an air passage, up to where the head starts.... The O-ring is captive on the screw, and snug enough to just pull into the "V" formed by the underside of the flat head screw and the valve body.... When you pump, the air stretches the O-ring to allow air to enter the valve.... The O-ring then pulls back into the "V" to seal.... The O-ring is the only moving part.... no springs or retainers are required....
Bob
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Crosman would also have to make a 5.75 cc valve with big ports, however, and a linkage strong enough for 2300 psi.... Neither are going to happen, IMO.... That pumping effort is just too much, and too hard on the gun.... You might be OK with it, but the average guy, no way.... Don't forget, Crosman recommend only 8-10 pumps....
Bob
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Crosman would also have to make a 5.75 cc valve with big ports, however, and a linkage strong enough for 2300 psi.... Neither are going to happen, IMO.... That pumping effort is just too much, and too hard on the gun.... You might be OK with it, but the average guy, no way.... Don't forget, Crosman recommend only 8-10 pumps....
Bob
I think the biggest limitation the 392 has is the width of the pump tube. Sometimes I think it's TOO wide. My 392 and 1300KT both do about 16 FPE - at 10 pumps in the 392 and 20 pumps in the 1300 - and it's actually easier to get there in the 1300, even though it's developing about 33% more pressure and has a shorter pump arm.
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Crosman would not go out there for sure.
To me they should add .20 and .25 to the lineup with no internal changes. I think the sales would be good.Crosman would also have to make a 5.75 cc valve with big ports, however, and a linkage strong enough for 2300 psi.... Neither are going to happen, IMO.... That pumping effort is just too much, and too hard on the gun.... You might be OK with it, but the average guy, no way.... Don't forget, Crosman recommend only 8-10 pumps....
Bob
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What weight pellet for the 392 at 16 FPE at 10 pumps?.... Any mods, or is that dead stock?.... I can start from there and predict what a .25 would do....
Bob
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What weight pellet for the 392 at 16 FPE at 10 pumps?.... Any mods, or is that dead stock?.... I can start from there and predict what a .25 would do....
Bob
Crosman rates the 392 with 14.3 g @ 685.
In reality they are 640 fps stock. 8 pumps maximum.
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14.3 grain pellets. The only modifications are a valve that is unscrewed by about 1/16" to bring it closer to the piston and to slightly increase volume, and a check that has been shortened by 3/16." Volume is about 4.55cc.
Completely stock it did 12.9 FPE at 8 pumps. With the modifications it does 14.9 FPE at 8 pumps and 16.3 FPE at 10 pumps.
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If a stock 392 with 14.3 gr. is 640 (13 FPE) at 8 pumps…. with no changes other than a .25 cal barrel, and using JSB Kings (25.4 gr.) I would expect about 540 fps (16.4 FPE)…. Personally, I think that is too slow for most guys, although it is still over the 500 fps limit in Canada.... ::)
If mild mods get you to 16 FPE at 10 pumps, that gun should hit about 600 fps with the 25.4 gr. in .25 cal (just over 20 FPE)…. IMO I'd sooner have a .22 cal shooting 15.9 gr. pellets at 680 fps.... Using the new Hades pellets it should perform as well as a .25 for hunting, with a far flatter trajectory.... I think the stock valve is just too small for a .25 cal.... JMO....
Do you use a larger CS O-ring when you unscrew the valve 1/16" ?.... I have an aluminum 392 valve here with a 0.102" CS O-ring, and when you screw the two halves together, you end up with about a 1/16" gap and can see the O-ring.... Is that how you did yours?.... What is the LOA of the valve, or the distance between the original 2 O-ring grooves?....
Bob
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That's exactly it... a -113 o-ring in the center with some teflon tape on the threads for good measure. There is little interference if any between the pump cup and the valve even with this adjustment. It's amazing how much pump efficiency was left on the table in the stock form. I guess Crosman/Benjamin really didn't want anyone to break the gun by over-pumping.
My valve is brass, but probably the same dimensions as the aluminum one you have. I don't have any measurements, though.
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the best thing that Bob, James perotti and Phil riggs did with their pumper designs was getting away from the constraints of the 392 main tube and soldered barrel design..James was the designer of a 63 fpe 32 caliber pumper many many moons ago.. Phil designed and still uses a 13.5 fpe SSP with 1.375 piston..Bob as we know used a disco platform ..
back to the conversation i just read, leaving the valve as a solid unit and adding a 16gram ( perpindicular ) threaded co2 cart would also add tremendous volume with little effort.
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heres a link to some of James work.. an ssp in this case.
https://www.tapatalk.com/groups/thegreencrosmanforum/here-is-james-perotti-s-ssp-pictures-stunning-t44499.html (https://www.tapatalk.com/groups/thegreencrosmanforum/here-is-james-perotti-s-ssp-pictures-stunning-t44499.html)
and the pumper
(https://live.staticflickr.com/65535/47969129487_c68065794a.jpg) (https://flic.kr/p/2g5SjYF)32 caliber pumper james (https://flic.kr/p/2g5SjYF) by murphyrobert9 (https://www.flickr.com/photos/56743574@N07/), on Flickr
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I think the first thing I would look at in a 392 is machining the front of the valve flat, shortening it to just ahead of the O-ring, and fitting a Flat-Topped Piston, similarly O-ringed.... all of that to eliminate headspace.... Inside, I would either fit a captive O-ring check valve (assuming there is enough length after shortening the front)…. or shorten the check valve and use the washer on the internal shoulder trick and fit a MUCH lighter check valve spring along with a shorter (and maybe lighter) valve spring .... This reduces pumping effort considerably because the stock spring pushes so hard on the check it takes a couple of hundred psi to crack it open, and that pressure never sees the inside of the valve....
If planning on exceeding 1500 psi, I would definitely use Mac1's High-Strength pump linkage.... If I wanted to use over 2000 psi, I would step down the diameter of the pump tube to that of a 1322, like I did in this conversion for a 2240....
(https://i378.photobucket.com/albums/oo221/rsterne/22%20Uber-Pumper/IMG_3840_zpsb7b93296.jpg) (http://s378.photobucket.com/user/rsterne/media/22%20Uber-Pumper/IMG_3840_zpsb7b93296.jpg.html)
Of course there would be a lot of scratch-building involved, and a much longer pump tube and custom linkage required.... but the pumping effort at 2300 psi would be the same as a 392 at 1500.... 3000 would be the same effort as 2000.... 8)
Bob
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So we would want a longer pump tube with a narrower 5/8" I.D. and perhaps a thicker wall wouldn't be a bad idea if one were building from scratch. Going longer actually makes a lot of sense to me when I consider that a 392 is a very compact rifle at about a foot shorter and 3-4lbs lighter than the typical magnum springer. The longer form factor, let's call it a magnum pumper for now, would also allow for a longer pump handle, increasing the shooter's mechanical advantage. So if the 392 has a stroke of about 7.5 inches and if the tube I.D. is reduced from ~.775" to .625" the stroke would need to be about 4 inches longer to match the 392's swept volume. At 6 inches longer, the Magnum Pumper would have 17% more swept volume. Now we're talking about a rifle that could easily achieve about 22-25 FPE on only 10 pumps. With the 6" longer tube, it would only be natural to mate it to a 26-inch barrel that ends flush with the end of the tube. Now we're getting very close to 30 FPE on 10 pumps, with the ability to pump it many more times, and it's still half a foot shorter than some of the break barrels on the market.
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In terms of strength, 3/4" OD x 0.065" wall tube (like the 1322 uses) is plenty strong enough for well over 3000 psi with a safety margin greater than 4:1.... because as you decrease the diameter you increase the thickness/diameter ratio, which governs the safe pressure.... However, if you want to turn the outside of the tube to fit inside the 0.775" tube on the 392, you will need to start with a larger OD.... You could use 20mm OD x 2mm wall and end up with a 0.630" ID, and a large enough OD to turn it to fit inside the 392 tube.... If you can only get US size tubing, you will have to use a 0.120" wall, as the OD will have to be 7/8", which would give an ID of 0.635".... With that much wall thickness, you could use 2024-T3 aluminum, which is available with that 0.120" wall.... or of course DOM steel tubing or 4130 CrMoly…. With a 0.775" OD and the remaining 0.070" wall, using 2024-T3 aluminum, at 3000 psi the safety margin is 3.0:1 to yield and 4.2:1 to burst....
Remember, however, that the ID of the 392 tubing is subject to whatever pressure you run, and that is the diameter of the valve and the end of the pump tube subject to the pressure, and hence the axial load on the fasteners.... I don't know the wall thickness or material of the 392 tube, so can offer no suggestion as to the MSWP of that tube, or the anchoring of the valve or pump tube.... I don't even know if you are operating with a sufficient safety margin at 2000 psi....
Bob
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extending the sweep on a pumper is not as easy as it seems.. the linkage geometry needs to be done in stages were the stroke to be way longer and the arc of the pump arm to not reflect that. In other words , it would need to be operated by a tall person with a long reach at some point.. 2 stage piston designs solve this but thats another bag of worms
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in other words, as the stroke distance increases in a conventional pumper design so does the guiding slot in which the linkage will reside. To exacerbate the issue is the point in Arm travel where pressure will begin to rise ( it happens earlier when the arm is at a higher angle in relation to the tube )
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one concept ive read about was skipping 70 % of the stroke distance and simply starting at the very end with a massive piston ( 2 inches diameter ) this way the 40 to 1 mechanical advantage( or more ) of the linkage is already in full swing when the stroke starts.. I have not seen it executed by the way LD described it it seemed quite viable.
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Those are good points. Being a big tall guy helps . I hope the balanced valve adds some finesse to my build. If there is a gray area where a faster off the seat could add any power would be nice. Before I tear it down, I should document the fps at pump 4 thru pump 14. I need to put clay on the hammer to measure lift of poppet at mid pressures. This may be needed to help in the new spring design.
Looking at replacing the white delrin spacer with a brass balance chamber. The check valve spring will be pocketed in the check valve. The balance chamber vent hole will be very small (#60) exit to throat area on angle instead of drilling thru stem.
Ideally the poppet return spring would be inside balance chamber if there is room.
Currently the throat is .236 with a .093 stem. Poppet diameter is .320.
Transfer port is .236 x .170
Bolt probe is .078
Check valve spring is 400 grams.
Return spring is 1300 grams closed.
The balance chamber o.d. is .373 so with a .05 wall a .273 i.d. is possible.
The metric 1 mm orings can be used.
Any ideas on the size of this end of the valve? I was thinking 5 millimeters.
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everything looks great.. Question.. would it be easier to make a new front end of the valve with the balance chamber built in ??i know the travel distances become super technical ,im just brainstorming aloud. another key factor is the stem length.. By shortening the stem a few MM you have a longer hammer travel ( more fpe from the hammer stroke
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studying rhe pic further i realized im not really grasping the arrangement.. maybe bob will chime in
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everything looks great.. Question.. would it be easier to make a new front end of the valve with the balance chamber built in ??i know the travel distances become super technical ,im just brainstorming aloud. another key factor is the stem length.. By shortening the stem a few MM you have a longer hammer travel ( more fpe from the hammer stroke
No I don’t think a new valve end would help. The removable chamber makes it a simple drop in replacement for the existing spacer. The way the stem length relates to the bolt can’t be changed but whatever you shorten the stem can be added to the hammer. The reduction of unneeded stem travel could allow the placement of the spring inside the balance chamber, the poppet could be shaped and turbulent airflow over the spring eliminated.
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will be interesting to see the numbers with other factors essentially the same.
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will be interesting to see the numbers with other factors essentially the same.
I have a few details to factor in but there is a wide window to work in. No need to work the details out in this thread. I am keeping my expectations low.
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It should be great for trigger feel and accuracy. Any improvement in velocity would be a bonus.
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I know this is an old thread but would any of y’all that have modded 392’s be willing to do some power mods on mine if I paid you? I reached out to mac1 several times but never got a response.
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Jake, if you emailed Tim McMurray, you probably won't get a response. You need to call him.